The present invention relates to a molded body (molded article) of a resin, which has open voids and good sound absorbing qualities in a wide frequency range, and possesses sound absorbing qualities useful for floor backing materials, wall backing materials and core materials thereof, automotive interior materials, or the like.
The conventional sound absorbing material is generally produced by forming an iron plate or the like into a box, providing openings for conducting sound absorption in the front surface thereof and containing glass wool therein. However, a sound absorbing and insulating panel of this kind has such drawbacks that (i) its weight is heavy and hence very difficult to install, (ii) glass wool used as a sound absorbing material has no water resistance, (iii) it is difficult to fabricate it without using other parts and the like, and its installation becomes large-scale, and (iv) reinforcing members as a sound absorbing member, a sound insulating member and a panel are different from one another, and so the cost, weight and production man-hour thereof increase.
Japanese Patent Application Laid-Open Nos. 137063/1995 and 108441/1996 describe the fact that expansion-molded articles with open voids obtained by molding foamed particles of a specific shape have sound absorbing qualities. These publications describe the expansion-molded articles as having excellent sound absorbing qualities, but this only means that the molded articles have excellent sound absorbing qualities in a narrow acoustic frequency range of specific wavelengths. Therefore, the publications have not taken achievement of an excellent sound absorption coefficient in a wide acoustic frequency range into consideration.
It is an object of the present invention to provide a molded article of a thermoplastic resin having excellent sound absorbing qualities in a wide frequency range.
Another object of the present invention is to provide a molded article of a thermoplastic resin, which has open voids and is excellent in air permeability in addition to sound absorbing qualities.
A further object of the present invention is to provide a molded article of a thermoplastic resin, which is excellent not only in sound absorbing qualities, but also in air permeability and heat insulating property.
According to the present invention, there is thus provided a molded article obtained by bonding small resin pieces composed of a thermoplastic resin to one another so as to form open voids. Examples of the form of the small resin pieces include particles, chips, crushed products, etc. In the present invention, one of these or a mixture thereof is used. The void content of the molded article according to the present invention is 10 to 60%, and the bulk density thereof is 0.01 to 0.6 g/cm3.
The small resin pieces in the present invention are preferably of a structure having through hole(s) or non-through hole(s), particularly a structure having through hole(s). However, the present invention is not always limited to those of the structure having such hole(s). The small resin pieces may be in various shapes. However, the small resin pieces are selected so as to give a porosity of 45 to 80% in a most densely packed state. When the small resin pieces are not of the structure having the hole(s), the porosity is defined as a proportion, in terms of percentage, of a volume occupied by the space portion given by spaces among the small resin pieces to the apparent volume of the small resin pieces in a most densely packed state. When the small resin pieces are of the structure having the hole(s) on the other hand, a volume occupied by the spaces of the holes is also calculated as the porosity. Therefore, the porosity of this case is defined as a proportion, in terms of percentage, of a volume occupied by the total space portion of a space portion given by spaces among the small resin pieces and a space portion given by the spaces of the holes in the small resin pieces to the apparent volume of the small resin pieces in a most densely packed state.
In the present invention, the porosity of the small resin pieces is 45 to 80%.
Incidentally, the reason why the void content of the molded article may be lower than the porosity of the small resin pieces in some cases when both of the porosity and the void content are compared with each other is that a part of the small pieces is melted, and the small pieces may be expanded in some cases when the small pieces are bonded to one another to obtain a molded article. The molded article according to the present invention features that it has not less than 3 continuous center frequency measuring points at which the sound absorption coefficient is at least 70%, and not less than 7 continuous center frequency measuring points at which the sound absorption coefficient is at least 50% in the sound absorption coefficients at the center frequencies of 250, 315, 400, 500, 630, 800, 1000, 1250, 1600, 2000, 2500 and 3150 (Hz) as determined by the measuring method of reverberant absorption coefficient prescribed in JIS A 1409.
In the present invention, the small resin pieces are preferably in the form of particles having through-hole(s). In this case, a ratio (L)/(D) of the maximum length (L) of the through hole in a direction along the hole thereof to the maximum diameter (D) of the small piece at the section perpendicular to the direction along the hole is preferably 0.7 to 1.2.
In the present invention, a polyolefin resin is preferred as the thermoplastic resin, and the small resin pieces are preferably formed from an expandable resin. The present invention includes the constitution that a molded article is bonded to a skin material. In general, the molded article and the skin material are bonded to each other by presetting the skin material in a mold, filling a number of the small resin pieces into the mold and heating the small pieces, thereby producing a molded article.
In the present invention, the small resin pieces having a porosity of 45 to 80% formed of a thermoplastic resin are used as a material and bonded to one another so as to form open voids, thereby forming a molded article. The void content of the molded article is 10 to 60%, and the bulk density thereof is 0.01 to 0.6 g/cm3. The molded article according to the present invention becomes excellent in sound absorbing qualities in a wide frequency range making such structural features a premise condition. Its sound absorption coefficient is greatly improved compared with the expansion-molded articles having mere open voids.
As described above, the molded article according to the present invention has excellent sound absorbing qualities, can exhibit a sufficient function even in the form of a simple material in an application field of sound absorption and sound insulation and moreover can be applied to other wide uses such as combinations with other sound absorbing materials. The molded article according to the present invention can be produced by a molding process comprising filling the small resin pieces into a mold and heating them, thereby integrally fusion-bonding the small resin pieces to one another.